Testing dielectric materials



June 14, 1949. M. A. ELLIOTT 2 Sheets-Sheet 1 www `lune 14, 1949,.

Filed July 13, 1945 M. A. ELLIOTT TESTING DIELECTRIG MATEIALSShetS-Sheet 2 RECEIVER .L E; E

R. E Auono OSCILl-.ATOR osclLLAToR l55 I so e CAPACITANOE CELL MEAsuRlNGUNIT 62 OSCILLOSCOPE gmc/rm MYRON A. ELLIOTT Patented June 14, 1949UNITED STATES PATENT OFFICE 2,472,814 TESTING DIELECTRIC MATERIALS MyronA. Elliott, Washington, D. C. Application July 13, 1945, Serial No.604,940

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370O. G. 757) 8 Claims.

This invention relates to a method of and apparatus for determining theelectrical characteristics of dielectric materials and more particularlyto a method of and an apparatus for determining the electricalcharacteristics of liquid dielectric materials. l l

Where dielectric materials are to be employed in electrical apparatus,it is necessary to ascertain the electrical characteristics of thedielectric materials and particularly among these the dielectricconstant and power factor. While, in the case of solid dielectricmaterials, conventionally utilized electrical tests provide asatisfactory indication ofthe dielectric constant and power factor, inthe case of liquid dielectric materials, however, testing by the methodsapplicable to the testing of solid dielectric materials may be bothinaccurate and impractical. A Anobject of the present invention is toprovide an effective and emcient method of and apparatus for thedetermination of the electrical characteristics of liquid dielectricmaterials.

In accordance with one embodiment of this invention, a quantity of theliquid to be tested is placed on an electrode surface, and the electrodeassembled in a cell so as to be spaced a predetermined distance from thesurface of a second electrode, sandwiching the liquid therebetween. Theelectrodes are connected to a measuring circuit whereby measurement ofthe capacitance and conductance o-f the test sample may be made. Theliquid is cooled by cooling the electrodes to a temperature below thepeak in the power factor curve. Heat is then applied to the cell to warmthe cell at a predetermined rate, measurements being made of the capaci-CFI tance and conductance at the beginning and at intervals during thetemperature rise. the test an alternating field of constant frequency isapplied to the liquid from a suitable frequency source.

Other objects and advantages of the present invention will be apparentfrom the following detailed description taken in conjunction with thedrawings, wherein: v

' Fig. 1 is a vertical, sectional view of a test cell assemblyconstructed in accordance with one embodiment of this invention;

Fig. 2 is a detail, e1evaucna1 view cf the base' of the upper electrodeof Fig. l;

Fig. 3 is a schematic view illustrative of a testing circuit used inaccordance with this invention; and

Fig. 4 is a perspective view of o fingers 36.

e of the spring During y The method of this invention is based on thefact that dielectric liquids show a characteristic variation ofdielectric constant and loss factor (anomalous dispersion) with changein temperature when the measurements are made While the liquid issubjected to high radio frequencies. Thus, by causing a material to bein a state of anomalous dispersion itis possible to differentiatebetween materials and to determine the characteristics of differentmaterials under the same conditions.

The common dielectric liquids fall into the general class of liquidswhich are polar but not ionized. When such liquids are placed in anelectrostatic field, the molecules tend to become oriented because ofelectrical interaction between the electric dipoles in the liquidsmolecules and the electric field. When this eld is alternating, themolecules tend to follow it and so are turned first in one direction andthen in the other. If the alternations of the field are sufcientlyrapid, the molecules can no longer follow, but remain in a random state.Under these circumstances the dielectric liquid has a low dielectricconstant and low loss factor. At intermediate frequencies, where themolecules can only become partly oriented, the dielectric constant hasan intermediate value, while the dielectric loss in the liquid is atamaximum or peak. At low frequencies,

. the dielectric constant reaches its highest value (static value) whilethe loss factor drops to a low value. Since the viscosity of the liquidmedium in which the molecules are suspended is a function oftemperature, it follows that the speed at which the molecules can orientin an alternating field also depends on the temperature. The temperaturecan, therefore, be used as the variable to obtain the anomalousdispersion curve while frequency is held constant. It will be understoodfrom the foregoing that the frequency employed must be such that thedata fall in an anomalous ldispersion range of the material under study.

Theoretically, it would be anomalous dispersion state possible to obtainthe by lowering the ternperature of the liquid sufficiently withoutapplication of a high frequency practical matter it would if notimpossible, to lower the temperature sui-liciently. Also some liquidswould freeze prior to reaching the anomalous dispersion state. ever, bythe application of a sufficiently high frequency, in practice on theorder of one to thirty megacycles, the temperature level required toproduce the anomalous dispersion state may be raised so that, in mostcases the anomalous disfield. However, as a be extremely diiicult,

HOW-

temperature of the liquid sample is varied, while-f the frequencyapplied is maintained substantially constant. It will beunderstood,lweverjthatit would be equally possible in accordance-Withthis invention to maintain the temperature constant and to vary thefrequency, or`t`o'iy`both.

Referring now to the drawings, wherein'isillustrated the apparatus ofthisinvention, antiparticularly to Fig. 1 thereof, it"`will* bes'enthattlie apparatus is mounted on a base `4, which may be annular inshape and made of a suitable electrical insulating material havinglowlect'rical losses, 'suche-s' polystyrene. Q 'theupperjsida as viewediii-Fig. 1, 'f tiiej basedis-rrioiirited' a flat, annularteii'ninalplateijwhihi's ina-de of an'elctrically iidiitive'nateriaLand'whichserves l'as a suplt i for r"a v.la'ge "cyliiid'ijically vsha'geldhousing 6, rountedl on the upp'er side 'thereof and suitablySer'd'fthereto, Thehusingf is intended prinifaily tofinsulatefthernlally the apparatus so t'iattheapparatuslocttl"thereinwill be unaiectedinsoiar as feasible byroom temperature. In plia'ctie,"it l'las `'been rfolnd 4 that 'certain resifus A"-inaterials, "si'lh fa"'plienol'io condensation poduts having low th r'r'ialconductivityprovide satisia'ctdry tlirrnall'ins'ulating" properties for thecrltainr.

The 'upper 'Tel'tlode i" 22 'has l"attritiched thereto a `-eylir'idriealf'el'ej'rrient YA20 -f resln'ous material, such 4`'-as al phenolic"condensation product which has lwithfmalfcdiidctivity. The'ele'rnent2'0 is provided at its' lower' endrwith e. cylindrical tap orplug-"2Unai/idg1 siibstari'tialiy the same `diametras the tube ZBnd thelowrpait ofthe 'electrode 22. -The elietrde`t 22 :and cap2l' areelectrieelly r'connected :by e ifedatirie "of electrically conductive"material 'such "-as `si1ver or "gold, a showing'. ofVwhichlfis"foini'tted 'from the drawing because of 'itsxtril'ieiihnes'scinparedto 'the dimensions of-'aidjacent parts. 'Itistobe understood that such coatingI extends -over 1 the i entire outer face`f' thelcylin'driridf n to andbver the pertion'eflthefadjater'itperiprierarsurfaees'f thefcap= 2 l andfeleetrodef 22. iFor' effectingelec- ,4 trical connection of the cap 2| with the socket l2 there isprovided a plurality of arcuate spring contact members I1 mounted attheir upper ends on a collar I5 and extended downwardly into the socketso as to resiliently engage the cap 2| when the latter is inserted inthe socket. In order to mount the cap-2l and electrode 22 .in the endsof the tubeZU', they mayibe=e ternlly threaded adjacent their bases to tinto and engage the lower and upper ends respectively of the tube 20,which maybetinternally threaded, as shown in Fie '1. AThe 'lectrode 22,which may be slightly itapered, :extends into and closely engages thesides of aV correspondingly tapered aperture 23 frr'nejd throughth'ecenter of the electrode supporting lcollar or ange I8 of insulatingmaterial, and the upper surface of the cap 22, as viewed in Fig. "1,liesi'nfthe same plane as the upper surface f'the flange I8. Theelectrode 22 is rigidly secul'ed to the flange I`8 as, for example, by asuitable'eemerit.

'Siiic'e th flange Iii'and'tile'electrode 22 will, in accordance withvthis invention, belocated in a region subject to "wide temperaturevariations, it is :desi'rable'thats they both be'nlade ofrnaterialshaving low coefcients of expansion. 'At the same time,"slncethe lil'ange8 is'to'js'erve'both as a support 'fdr the eleejtrdde zzjandto insulatethe electrode '22 :from *the adjacentfapparatus, it is desirable that`the insulating material "of which thedise la-isiirade leerme which haslow 'ele-et'rical losses. In" practice,"v it'i'l'as been found 'thatquartz prcvidesfa satisfactory 'meterleife'r 'the disc I8 and thatInvalgrsirice it l"has j a "coefficient df expalisieril substantially.identical Awithv that of quartz, wliieiris veryl'o'w, is 'satisfactorymaterial for the electrode "22. When assembled in the appai1atus,"theupper n'd oftlfle electrode 22 and' the upper'side lofthe associatediiang'e B present'. fa srriooth,4 perfectly ilat, circular 'surface tothejunderside'of a second electrode 25fwhich'niayb'e' o'fdisc'shape, asshown. The electro'de 2 5 ,"th`e: 1ower 1s'ijdfe of which is shownin'Fig. 2,'lias integrally formed' 'therewith'a wide circular-'boss"26,"Iofcated substantiallyin the 'centralportion'of'the unfdersidethereof,"and extending downwardly'therefrom 'a vshort distance, to form,irrieiffect, a secondnat fdisc mounted on the electrode' 25. r'lhe bosshas formed thereina lcen'- trally"located;'circulan'recessfor chamber 21of a suitable depth to provide the'necess'ary spacing between ltheelectrodeszz arides. The optimum depth depends on the t type fofmaterial being tested f and `rmay vary considerably accordingly withdifferentlrriate'rials. jg'ihelo'ase er the recess 21' :is made `flatso'that"tvhen the'v electrode 22 is assembled-against `theeleetredez'strie upper en'd surface of tige eleetjrddeffzzwill-lie inge'plane parallelfto the plan'ef t-hejbase'ofv the recess 21.l 'It will'befrretedthat tnewid'tlrdr the recess zlis made substantially greaterthan `the width, for diainet'en; 'or the upperisuiface' of theelectrode' 22 sdth'at only the quartzlajnge' l8"directly4 contactstnebossizirferrelectrodefzaitmispreventirig elec'- trical connectionbetween theelectrode i 6 and the electrode 25.

in pla'eing-'fefsaiiiplef-ef the liquid te lee-tested" iii the recess 21, thefjeleetrddeszz andzsare retrieved from ithe 4container At,dissassembled, and theelectrode'25 inverted softhat'therecess 2l 'maylperilled. --Aj^quaritityertne liquid tote te'stdis'placedintherecessZ'ifa suicientquan tityA Ybengused to completely ll the recess'21. spaced circulangroovesZB are"formed adjacent the recess 27, and areconcentrically disposed ame-,14

with respect thereto. These grooves receive any excess liquid when thetwo electrodes are sand'` wiched together, slots 29 being formed betweenthe recess 21, the grooves 28 and the outside of the boss 26, as shownin Fig. 2, to permit the material to pass therebetween. It is desirablethat sufficient liquid be used to completely fill the grooves as well asthe recess. By suitably radially spacing the slots 25, the liquid as itflows from the recess to the grooves may be caused to travel in acircuitous path and thus when the liquid contracts due to cooling, theliquid in the grooves serves as a reservoir and will reenter the recessthus keeping the recess filled at all times and preventing air fromentering the rec-ess and conversely when the liquid expands it will beable to leave the recess 2l through the slots 29 and grooves 28.

After a suitable quantity of liquid has been placed in the recess 21,the electrode 22 is placed thereover, causing the outer surface of theflange I8 to flatly engage the portions of the boss 25' intermediate thegrooves 28 on the underside of the velectrode 25, and sealing the recessEl, except for the slots 29. The electrode 22 and flange l8 are thenresiliently clamped to the electrode 25 by a ring or collar 32 which isdisposed about the periphery of the flange i8 and may be secured to theelectrode 25 by a number of threaded membersA 33 which extend throughthe ring 32 and threadedly engage the electrode 25. Knurled knobs 35 areprovided on the lower ends of the threaded members 33 to facilitateassembly. In order vto provide a resilient support for the electrode22,a number of spring ngers 36 are mounted on the ring 32 and extendradially inwardly' from the under side thereof. These fingers may beprovided with lugs 3'! on the upper sides of their inner ends which areIcaused to bear against' When the electrode assembly described in the`preceding paragraph is positioned in the container the cap 2l extendsinto the tube vl2 and is resiliently engaged by the springcontactmembers l?.

low thermal conductivity similar to the material of tube 2li, is coatedwith an electrical conducting material such as gold or silver, a showingof which is omittedfrom the drawing because of its eX- treme thinnesscompared with the dimensions of adjacent parts. It is to be understoodthat this.

coating extends over the entire outer and inner surfaces of the member4t and over adjacent portions of the'collar 4l and the terminal annulus5. The collar il is formed of a material having good electricalconductivity, such as brass or cop. per, and thiseollar is rigidly xedto the upper end of the tube te by a number of transverse screws 44. Inpractice it has sometimes been found desirable to coat not only the tube4E) but also the collar 4l and at least the area of the disc 5 adjacentthe lower end of the tube 45 with electrically conductive material toprovide as low an electrical resistance path as possible.

When the electrodes are assembled together and the vcap 2l is properlypositioned in the tube l2,. the lower peripheral ledge of the electrode25,

For effecting electricalconnection be tween the electrode 25 and theterminal annulus4 5, the tube which is made of a material having fcontact. It isralso desirable that the .electrode and boss 26 be coatedon its under side with electrically conductive material. Because theboss 25 comes into direct contact with the liquid a coating of amaterial substantially unaffected by possible corrosive action of theliquid is preferable. Gold has been found to be a suitable material. Itwill be apparent that by providing the intermediate tube of low thermalconductivity material, conduction of heat or cold from the electrode 25is minimized.

In order to further thermally insulate the region' wherein the testsample is located, a plurality of annular discs 135 are located in thearea between the lower side of the fiat annulus IB and the upper side ofth-e disc I0 and are supported in spaced relation, one above the other,by a sleeve 46 which extends axially through the apertures formed in thecentral portions thereof and is suitably xed to the discs. The internaldiameter of the sleeve d6 is made such as to permit the element 2li topass therethrough preferably without contacting. The peripheries of thediscs should extend to the inner wall of the sleeve 45 and may be sealedthereto. Because of the high frequencies at which this apparatus isintended to operate, it will be understood that it is desirable that thediscs 45 and sleeve Mi be made of a material having in addition to thecharacteristic of low thermal conductivity, also low electrical losses.Polystyrene been :found satisfactory. rBy providing the dead air spaces4l between the discs 45, rather than using one solid disc of thecombined thickness, it has been found that somewhat better thermalinsulation is obtained.

In order to subject the test sample to the wide range of temperaturevariation which it has been vfound desirable to employ, it has beenfound satisfactory to .first cool the test sample and then to permit thetemperature to gradually rise, possibly aided by the application of heatfrom a heating element. In order to cool the test sample an annularbucket 49, which is preferably made of metal in order to have high heatconductivity, isV filled with a coolant material, such as solidifiedcarbon dioxide or ice, and is positioned against the upper surface ofthe electrode 25, as shown inv Fig. 1. Since both the electrode 25 andthe bucket le are made of metal, because of the rapid heat transferbetween contacting metals, it will be understood that the electrode 25will approximate the temperature of the base of the bucket 49, andconsequently the liquid in the recess 21 will be cooled to substantiallythe same temperature as 4the electrode 25.

In order to confine and insulate the bucket 49 from the roomtemperature, insofar as feasible, a` large tube 5B of but slightly lessdiameter than the `container 5 and made of a material having low thermalconductivity, such as Bakelite or thermal conductivityy may bepositioned on the upper surfaceof the container 5, as shown in Fig. 1,to close the container and thus to furtherI insulate the electrode andbucket from the external temperature.

`In order to indicate the temperature of the electrode 25, a supportingtube 52 is mounted on A cover or lid 5I of disc shape and also made of amaterial having l0w-v sa-tam theuppsrgsiue er .the electrode 25 andaligned substantially'withlthe axis thereof and an aperture' 54 is:formed in the cover plate f 5 I to permit a suitable thermcco'uple v.ora theimometer '55 of (shape 'to be lowered therethrough into vthe tube52 'and 'to have its "lower en'd 'bearra'gainst the rsides of a suitableArecess 56 formed 'in the upper-surface o'f the 'e1e'ctro`de`25 andconnected to the lower end df'theztube lthus bringing the sensitive'portion ofthe thermometer `as close as possible to the recess 27'wherein'the test 'sample islOcated.

Intheoperati'on ofthis apparatus, after a quantity 'of the test 'samplehas been placedv in 'the recess ZTI formed inthe electrode 25 'and theelectrode 22 has been a'clamped fthereto, as hereinbefore described,Ythe electrode assembly ispositioned iin the container 6 :andelectrically connectedto a 'suitable .capacitance measuring u'iitlll-,asshown in f3, orreleadtherefro-m being connected to a terminal'imounted on theouter side of the base "e .and suitably 'connected -`to'the lower end o'f the'tube 'i2 and thereby connected to Jtheelectrode22. The other lead from the unit "is 'connectedlto Ya'tel'm'inal 52mounted ori the disciadjacentthe terminal 6 I .and .is .connectedthrough th'e'coatedftube itothe Velectrode .25, as hereinbeforeidescribed.

While the dielectric 'properties may be `determinedbytany suitable'capacitance measuring apparatususingfthe'method and apparatus of thisinvention, asdescri'bedjit has'been found in prac'- tice 'that 'ja highdegree of 'accuracy and facility may 'be obtained :by employing a:suitable bridge type. capacitance measuring' unit, such as theTwin-Timpedance-measuring circuit described in The Proceedings 'of the.I. 'R."E.,'vo1. .28, pages 3 10-131'8, 'July `1940. `Suchunit'isindicated inthe block diagram Avof Fig. '3 by the numeral 60. 'Themeasurements .are cafrried "out by applying a high frequency signal,:for example, a signal 'on 'the orderof `10 megacyclesjfroma suitableradio 'frequency 'osci1lato1"6`4, .modulated by an audio vfrequencysignal :from an audio frequency oscillator 65, to the 'capacitance.measuring `unit 60, and through the `capaciitance lmeasuring Yunittothe electrodes Ifand-25. A receivericonnectedto the capacitancemeasuring unitldetects the audio frequency modulation an'd'applies 'the"detected audio sig'nalto one .pair of 'delectingiplates of anoscilloscope 31, `the o'therpair of 'deflectingplates ofivhichis'connected'to'theaudio oscillator. The

purpose o'ius'ing a test 'signal "of audio Vfrequency is to increase'the sensitivity of -the cathode ray oscilloscope.

In using this testing apparatus Vwith "thedescribed-cel1,thecellis"cooled, asfhereinhe'fore described, to thelowest "temperature "ofthe range over which thetest' istob'ejma'de andthe adjustment of the capacitance' bridg'emeasuring `unit required fto`obtain a fpattern'i'rrdicatingia balance offthebridge onfthe'oscilloscope is noted. Aft'er the `audio oscillator-nas'beenadjusted-to the proper l*frequency and 'the bridge balanced, thereshould appear a crescent-shaped pattern on the oscillograph screen. Anylvariation in 'Vthe'cap'aitance balance 'oftheibridge-willcause thecrescent to 'become fatter, while `'anyvariation in theconductancebalance willcaus'e the-crescent'to tip one way 'or theotherdep'ending'on'whichway the conductancebalance is-'oi. This ability`to 'distinguish fbetween capacitance @and 'conductance un'- balanceindependently while measuingthemo'- mental-.y electrical Vproperties ofa substanceithat is Ichanging temperature steadily *is of great :im-

graph, the characteisticcurvesfor dielectriccon starrt andpowerfactoriof .the .testmaterial may be obtained. Byma'king severaltests atdifferent applied frequencies any .desired .numberof such curvesmay be obtained.

Whereherein the .various parts .of the invention 1lxavje 'been referred.to .as `being .located iin an 'upper or 'lower position, it .will yloeunderstood 'that .this .done .solely .for .the .purpose `of .fa-

cilitating `description `and the references .relate only to 4therelative .positions of #the parts 'as shown 'in the accompanyingdrawings.

'While 4but one embodiment of this `invention `has beeen shown anddescribed, it-will be-.understood'that ,many' changesand modificationsmay beir'nade therein without departinglfrom thespirit oigscope. of'thepresent invention.

The invention vdescribed )herein ,may be manufactured and used .by or.for -theiGovernmentof the `United States .o'f America for--governmental purposes without Athe .payment of yany .royalties.thereon or therefor.

What'is claimed-is: "1. In 'an v'apparatusfory determining thedielectric `charact'eristics .of a liquid, `a l.pair of -spaced'electrodes having tat parallel opposed surfaces, one .of said electrodeshaving a `recess lformedA in `aace.thereof opposedrto .theotherelectrode for receiving .a l.quantity .of liquid, the lother of--said `electrodes being .o'f Arelatively small rvolume vand provided.with a current .lead comprising@ tube of .a material .liaving .lowthermalconductivity with a coating .of electrically I,conductivematerial, said other .electrode being, provided rwith'a supporting`collar .of electrically :insulating material'lh'aving .a .at face .ushrwith `the electrode face, means .for clamping saidelectrodestogether to.cause Said collar face -to Abear against the opposed "face .of saidrs't electrode, means for cooling said. liquid whilefinsaid recess.means .for applying V.an .alternating .field -to said electrodes, and.electrical-testingimeans operativelvconnectz ed withfsaid electrodes.

'2. 'In .an apparatus .for Adetermining i dielectric characteristics of.a liquid, 'a ybase -pf 'insulating material, ra metallic terminal,annulus -mourited on sa'idibasaia .current .leadelement infthe form of.a suppporting tube `of low thermal-conductivitymountedon=the=terminalr-annulusand having ia thin .coating ,of electricalconducting Vmaterial in A'electrical.contact with the-terminabannulus,.a .metallic .electrode .supporting collar mounted lon 'the .upperend..of -the current -lead in electrical contact with ,the said coating,an upper electrode Supported .onsaid collar intelecti'i'cal contacttherewith, said upper .electrode be- .ing of general disk shapahaving.onits .underside a 'central circular flatfacedprojection,saidprojectionhaving a central .circular Yrecessed .portion with va'fiat rear surface'parallelto the=said,flat faced *projection .and avlpluralitv vof .annular .grooves-communicating ,withtherecessedtportion and outside 'space'thrugh connecting grooves, a

9 lower electrode having a flat circular surface par- `allel to the saidfiat rearsurface of the recessed Lportion of the upper electrode,-' acollar of insulating" material secured to said low-er .electrode andhaving al flat upper face lying in the plane of the flat face ofthelower electrode, means resiliently clamping said collar and said upperelectrode together face to face concentrically to h old the -face of thelower electrode in-parallelism with the rear faceof the recessed portionof the upper electrode and spaced therefrom a distance equal to thedepth of the recessed portion, a' .centrally perforated supporting diskof insulating material mounted in the central opening of said terminalannulus,l a tubular metallic contact socket mounted in the centralperforation of the supporting disk, a second current lead elementsecured at one end to the lower electrode formed of material of lowthermal conductivity and coated with electrically conducting material inelectrical contact with the lower electrode, a metallic cap carried bysaid second current lead at the other end in electrical contact with thecoating and engageable with said contact socket, a sleeve ofelectrically insulating material of low thermal conductivity looselysurrounding said second current lead, a plurality of centrallyperforated insulation disks extending radially between said sleeve andsaid first lead element to form a closed thermal insulating cell betweenthe electrode and the remote ends of their respective current leads andthe terminal annulus and terminal socket, said upper electrode having athermometer recess in the top side thereof coaxial with the saidcircular recess portion in the lower side, a thermometer guide-tubeextending upwardly from the thermometer recess, an annular bucketfitting loosely around said tube with the bottom of the bucket restingupon and in thermal contact with the upper surface of the upperelectrode, a tubular jacket of low thermal conductivity surrounding thebucket and spaced therefrom, a cylindrical housing element of low.thermal conductivity supported on said terminal annulus and enclosingthe current leads, collar and jacket and a cover plate for the top ofsaid housing having a central aperture registering with the guide-tubeto permit insertion of a thermometer.

3. In an apparatus for determining dielectric characteristics of aliquid, a lower electrode having a flat surface, a collar of insulatingmaterial secured to said electrode having a, flat face lying in theplane of the flat face of the electrode, an upper electrode of generaldisk shape having a iiat Linder side face provided with a recessedportion having a ilat rear surface parallel to said flat under sideface, said upper electrode having in its flat under face a plurality ofannular grooves communicating with the recessed portion and outsidespace, and a metallic supporting collar for said upper electrode inelectrical contact with said electrode and out of electrical contactwith said lower electrode.

4. In an apparatus for determining; dielectric characteristics of aliquid, a lower electrode having a fiat surface, a, collar of insulatingmaterial secured to said electrode having a fiat face lying in the planeof the hat face of the electrode, an upper electrode of general diskshape having a flat under side face provided with a recessed portionhaving a flat rear surface parallel to said flat under side face, saidupper electrode having in its flat under face a plurality of annulargrooves communicating with the recessed portion and out- 10 sidespace,and a metallicy supporting collar for said upper electrode in electricalcontact with said electrode and out of electrical contact with saidlower electrode, together with'a pair of current lead supportingelements one for each electrode each comprising 'a supporting element oflow thermal conductivity coated lwith electrically conductive material.

5. In an apparatus for determining dielectric characteristics of aliquid, a lower electrode having a flat surface, a-collar of insulatingmaterial secured vto said electrode having a flat face lying in theplane of the flat face ofthe electrode, an upper electrode of generaldisk shapehaving a fiat under side face provided with a recessed portionhaving a flat rear surface parallel to said ilat under side face, saidupper electrode having in its flat under face a plurality of annulargrooves communicating with the recessed portion and outside space, and ametallic supporting collar for said upper electrode in electricalcontact with said electrode and out of electrical contact with saidlower electrode, together with a removable coolant-containing bucketiitted into thermal contact with the upper face of said upper electrode.

6. In an apparatus for determining dielectric characteristics of aliquid, a lower electrode having a flat surface, a collar of insulatingmaterial secured to said electrode having a i'lat face lying in theplane of the fiat face of the electrode, an upper electrode of generaldisk shape having a at under side face provided with a recessed portionhaving a flat rear surface parallel to said flat under side face, saidupper electrode having in its flat under face a plurality of annulargrooves communicating with the recessed portion and outside space, and ametallic supporting collar for said upper electrode in electricalcontact with said electrode and out of electrical contact with saidlower electrode, together with a thermometer guide-tube extendingupwardly from the upper electrode and a removable annular bucket ttedaround the guide-tube spaced radially therefrom and in thermal contactwith the upper face of the upper electrode.

7. In an apparatus for determining dielectric characteristics of aliquid, a lower electrode having a flat surface, a, collar ofinsulatingmaterial secured to said electrode having a at face lying inthe plane of the nat face of the electrode, an upper electrode ofgeneral disk shape having a flat under side face provided with arecessed portion having a flat rear surface parallel to said at underside face, said upper electrode having in its llat under face aplurality of annular grooves communicating with the recessed portion andoutside space, and a metallic supporting collar for said upper electrodein electrical contact with said electrode and out of electrical contactwith said lower electrode, together with means resiliently clamping saidcollar of insulating material and said upper electrode together.

8. In an apparatus for determining dielectric characteristics of aliquid, a lower electrode having a iiat surface, a collar of insulatingmaterial secured to said electrode having a flat face lying in the planeof the flat face of the electrode, an upper electrode of general diskshape having a, fiat under side face provided with a recessed portionhaving a flat rear surface parallel to said flat under side face, saidupper electrode having in its at under face a plurality of annulargrooves communicating with the recessed portion and outside space, and ametallic supporting collar for said upper electrode in electricalcontact with said elec- 1 1 trode and out of electrical contact withsaid lower electrode, together with a tubular. supporter inw thermal.conductivity for the-.upper electrode surrounding, the lower electrode,a, support.- of low conductivity for the lower electrode,Y a,- sleeve ofelectrical insulatingl material of low thermal conductvity looselysurrounding saidlowei: electrode support,- a. plurality of. centrallyperforated. insu'- lation disks extending radially betweemi said. sleeveand, said tubular support. to form al closed. thermal insulating cell.

, REFERENCES CITED The. 'fol-lowingx references aret of record'l in: thefile of this patent:

l2 UNITED STATES PATENTS Number Name' Date 2,126,363 White'et al Aug.9,1938 178,225 Diehl* et al'. Oct. 31', 1939 2,362,428 Biggs et al Nov;T, 1944 FOREIGN PATENTS.

Number Country Date 11633682.l Great' Britain' May 23, T922 2683651Great Britain Sept. 2H', 15937 OTHER REFERENCES Fields.Elecm'icalzEngineering; Trszns.,. Senta. 1941"J vol. 60, pp' 890,895'.

Sinclair, .Reprint Nw.. A48 ci. Engineering Dept; oli General Radio Coz.trom- Prom. of' the Institute of. Radio Engineers, July 1940,:pp.51m-318;4

